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Thermodynamics of Formation of Coffinite, Usio4

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Thermodynamics of Formation of Coffinite, Usio4 Thermodynamics of formation of coffinite, USiO4 Xiaofeng Guoa,b, Stéphanie Szenknectc, Adel Mesbahc, Sabrina Labsd, Nicolas Clavierc, Christophe Poinssote, Sergey V. Ushakova, Hildegard Curtiusd, Dirk Bosbachd, Rodney C. Ewingf, Peter C. Burnsg, Nicolas Dacheuxc, and Alexandra Navrotskya,1 aPeter A. Rock Thermochemistry Laboratory and Nanomaterials in the Environment, Agriculture, and Technology Organized Research Unit, University of California, Davis, CA 95616; bEarth and Environmental Sciences Division, Los Alamos National Laboratory, Los Alamos, NM 87545; cInstitut de Chimie Séparative de Marcoule, UMR 5257, CNRS/CEA/Université Montpellier/Ecole Nationale Supérieure de Chimie de Montpellier, Site de Marcoule, 30207 Bagnols sur Cèze, France; dInstitute of Energy and Climate Research, Nuclear Waste Management, Forschungszentrum Jülich GmbH, 52425 Jülich, Germany; eCEA, Nuclear Energy Division, RadioChemistry & Processes Department, BP 17171, 30207 Bagnols sur Cèze, France; fDepartment of Geological and Environmental Sciences, School of Earth Sciences, Stanford University, Stanford, CA 94305; and gDepartment of Civil and Environmental Engineering and Earth Sciences, University of Notre Dame, Notre Dame, IN 46556 Contributed by Alexandra Navrotsky, April 20, 2015 (sent for review February 10, 2015) Coffinite, USiO4, is an important U(IV) mineral, but its thermody- The precipitation of USiO4 as a secondary phase should be namic properties are not well-constrained. In this work, two dif- favored in contact with silica-rich groundwater (21) [silica − ferent coffinite samples were synthesized under hydrothermal concentration >10 4 mol/L (22, 23)]. Natural coffinite samples conditions and purified from a mixture of products. The enthalpy are often fine-grained (4, 5, 8, 11, 13, 15, 24), due to the long of formation was obtained by high-temperature oxide melt solu- exposure to alpha-decay event irradiation (4, 6, 25, 26) and are tion calorimetry. Coffinite is energetically metastable with respect associated with other minerals and organic matter (6, 8, 12, 18, ± to a mixture of UO2 (uraninite) and SiO2 (quartz) by 25.6 3.9 kJ/mol. 27, 28). Hence the determination of accurate thermodynamic Its standard enthalpy of formation from the elements at 25 °C is data from natural samples is not straightforward. However, the −1,970.0 ± 4.2 kJ/mol. Decomposition of the two samples was char- synthesis of pure coffinite also has challenges. It appears not to acterized by X-ray diffraction and by thermogravimetry and differen- form by reacting the oxides under dry high-temperature condi- tial scanning calorimetry coupled with mass spectrometric analysis tions (24, 29). Synthesis from aqueous solutions usually produces of evolved gases. Coffinite slowly decomposes to U3O8 and SiO2 UO and amorphous SiO impurities, with coffinite sometimes 2 2 CHEMISTRY starting around 450 °C in air and thus has poor thermal stability in the being only a minor phase (24, 30–35). It is not clear whether ambient environment. The energetic metastability explains why cof- these difficulties arise from kinetic factors (slow reaction rates) finite cannot be synthesized directly from uraninite and quartz but or reflect intrinsic thermodynamic instability (33). Thus, there canbemadebylow-temperatureprecipitation in aqueous and hy- are only a few reported estimates of thermodynamic properties drothermal environments. These thermochemical constraints are in of coffinite (22, 36–40) and some of them are inconsistent. To accord with observations of the occurrence of coffinite in nature resolve these uncertainties, we directly investigated the ener- and are relevant to spent nuclear fuel corrosion. getics of synthetic coffinite by high-temperature oxide melt so- lution calorimetry to obtain a reliable enthalpy of formation and uranium | coffinite | USiO4 | U(IV) minerals | calorimetry explored its thermal decomposition. Results and Discussion n many countries with nuclear energy programs, spent nuclear Ifuel (SNF) and/or vitrified high-level radioactive waste will be We used two independently prepared coffinite samples. A phase- disposed in an underground geological repository. Demonstrat- pure coffinite sample prepared at Institut de Chimie Séparative ing the long-term (106–109 y) safety of such a repository system is de Marcoule (ICSM), France is labeled “coffinite-F” and a less a major challenge. The potential release of radionuclides into pure material prepared at Forschungszentrum Jülich, Germany is the environment strongly depends on the availability of water and the subsequent corrosion of the waste form as well as the Significance formation of secondary phases, which control the radionuclide solubility. Coffinite (1), USiO4, is expected to be an important Coffinite, USiO4, is an important alteration mineral of uraninite. alteration product of SNF in contact with silica-enriched ground- Its somewhat unexpected formation and persistence in a large water under reducing conditions (2–8). It is also found, accompa- variety of natural and contaminated low-temperature aqueous nied by thorium orthosilicate and uranothorite, in igneous and settings must be governed by its thermodynamic properties, metamorphic rocks and ore minerals from uranium and thorium which, at present, are poorly constrained. We report direct calo- sedimentary deposits (2, 4, 5, 8–16). Under reducing conditions rimetric measurements of the enthalpy of formation of coffinite. The calorimetric data confirm the thermodynamic metastability in the repository system, the uranium solubility (very low) in of coffinite with respect to uraninite plus quartz but show that it aqueous solutions is typically derived from the solubility product can form from silica-rich aqueous solutions in contact with dis- of UO2. Stable U(IV) minerals, which could form as secondary solved uranium species in a reducing environment. These con- phases, would impart lower uranium solubility to such systems. straints on thermodynamic properties support that coffinitization Thus, knowledge of coffinite thermodynamics is needed to con- in uranium deposits and spent nuclear fuel occurs through disso- strain the solubility of U(IV) in natural environments and would lution of UO2 (often forming hexavalent uranium intermediates) be useful in repository assessment. followed by reaction with silica-rich fluids. In natural uranium deposits such as Oklo (Gabon) (4, 7, 11, 12, 14, 17, 18) and Cigar Lake (Canada) (5, 13, 15), coffinite has Author contributions: X.G., S.S., A.M., S.L., N.C., C.P., S.V.U., H.C., D.B., N.D., and A.N. been suggested to coexist with uraninite, based on electron probe designed research; X.G., S.S., A.M., and S.L. performed research; X.G., S.S., A.M., S.L., N.C., C.P., S.V.U., H.C., D.B., R.C.E., P.C.B., N.D., and A.N. analyzed data; and X.G., S.S., microanalysis (EPMA) (4, 5, 7, 11, 13, 17, 19, 20) and trans- A.M., S.L., N.C., C.P., S.V.U., H.C., D.B., R.C.E., P.C.B., N.D., and A.N. wrote the paper. mission electron microscopy (TEM) (8, 15). However, it is not The authors declare no conflict of interest. clear whether such apparent replacement of uraninite by a cof- 1To whom correspondence should be addressed. Email: [email protected]. finite-like phase is a direct solid-state process or occurs mediated This article contains supporting information online at www.pnas.org/lookup/suppl/doi:10. by dissolution and reprecipitation. 1073/pnas.1507441112/-/DCSupplemental. www.pnas.org/cgi/doi/10.1073/pnas.1507441112 PNAS Early Edition | 1of5 Downloaded by guest on September 27, 2021 labeled “coffinite-G.” In both cases, considerable purification af- ter initial synthesis was done to separate impurities (SI Appendix). The purpose of measuring and comparing both samples was to test whether consistent thermochemical data could be obtained on ma- terials prepared and purified independently in different laboratories. Before calorimetric measurements, samples were characterized by TEM, infrared spectroscopy (IR), powder X-ray diffraction (XRD), simultaneous differential scanning calorimetry and ther- mogravimetric analysis (DSC-TG) coupled with mass spectromet- ric analysis of evolved gases (MS-EGA), and by EPMA. The details are given in Experimental Methods and in SI Appendix. The XRD patterns (SI Appendix, Fig. S1) show the reflections expected for a zircon-type structure (space group I41/amd). Lattice parameters are a = b = 6.983(3) Å and c = 6.263(4) Å for coffinite-G, and a = b = 6.990(1) Å and c = 6.261(1) Å for coffinite-F. Crystallite size was estimated from X-ray peak broad- ening (65 nm for coffinite-G and 85 nm for coffinite-F). This observation is consistent with previous reports (8, 24, 26, 34, 40). Although the particle size may affect the thermodynamic prop- erties, we did not investigate this further. Even though coarse- grained but impure coffinite (>10 μm) has been documented from the Grants uranium region, New Mexico (8, 41), Colorado (8, 10), and hydrothermal deposits in Czech Republic (42), natural coffinite and materials produced in SNF alteration are usually fine-grained, having similar particle size as our synthetic sam- ples. So, the data obtained here are applicable to most “real world” situations. TG (Fig. 1) indicates a negligible amount of water in sample coffinite-F, not quantifiable from the MS trace. This observation agrees with Raman spectra on the same sample which show no signal associated with water (43). The water content of coffinite-G quantified by MS-EGA is 0.37 mol H2O per mole of USiO4 (slightly less than that estimated from TG analysis: 0.43 mol H2O per mole of USiO4). The water signal from MS suggests that there may be two water bonding sites, one with release near 150 °C and the other around 275 °C. The second peak (Fig. 1C)hasahigh- temperature tail up to 450 °C, suggesting some water molecules are strongly interacting with the sample. Above 450 °C, both samples oxidize and slowly decompose in air, as indicated by weight increase on the TG trace due to oxi- dation of U(IV).
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